1. Field of the Invention
The present invention relates to a magnetic recording medium and a magnetic storage apparatus.
2. Description of the Related Art
Recently, demands to increase storage capacities of HDDs (Hard Disk Drives) are increasing. As one means of satisfying such demands, a heat-assisted recording method and a microwave-assisted recording method have been proposed.
The heat-assisted recording method performs recording with respect to a magnetic recording medium using a magnetic head mounted with a laser light source, by heating the magnetic recording medium by the magnetic head. The microwave-assisted recording method performs recording on the magnetic recording medium by applying a high-frequency magnetic field from the magnetic head.
The heat-assisted recording method can reduce the coercivity of the magnetic recording medium by heating the magnetic recording medium and enable use of a magnetic material having a high crystal magnetic anisotropy constant Ku (hereinafter also referred to as a “high-Ku material”) for a magnetic layer of the magnetic recording medium. For this reason, the magnetic grain size of the magnetic layer can be reduced while maintaining thermal stability, and a surface recording density on the order of 1 Tbits/inch2 can be achieved.
On the other hand, the microwave-assisted recording method can perform the recording with respect to the magnetic recording medium with a recording magnetic field lower than or equal to the coercivity of the magnetic recording medium, by the assistance of the high-frequency magnetic field generated from an STO (Spin Torque Oscillator) mounted on the magnetic head. For this reason, similarly as in the case of the heat-assisted recording method, the microwave-assisted recording method can use a high-Ku material for the magnetic layer of the magnetic recording medium.
The high-Ku material includes ordered alloys, such as L10 type FePt alloys, L10 type CoPt alloys, L11 type CoPt alloys, or the like.
In addition, in order to isolate (or separate) crystal grains of the ordered alloy, the magnetic layer is added with a grain boundary material, such as an oxide including SiO2, TiO2, or the like, or C, BN, or the like. By employing a granular structure in which the magnetic crystal grains are separated at the grain boundary, an exchange coupling between the magnetic grains is reduced compared to a case in which the grain boundary material is not added, and a high medium SNR (Signal-to-Noise Ratio) can be achieved.
On the other hand, a write characteristic or an overwrite characteristic can be improved by forming a soft magnetic underlayer (or soft magnetic back layer).
Japanese Laid-Open Patent Publication No. 2009-158053, for example, proposes using a FeCoTaZr amorphous alloy for the soft magnetic underlayer. In addition, Japanese Laid-Open Patent Publications No. 2010-182386 and No. 2011-146089, for example, propose using a soft magnetic underlayer having a crystalline structure of NiFe, CoF alloy, or the like, or using a soft magnetic underlayer having a microcrystalline structure of FeTaC alloy or the like. In the magnetic recording medium that uses an L10 type FePt alloy for the magnetic layer and has a high Ku value, a substrate is generally heated to a temperature of 500° C. or higher, in order to improve the ordering of the L10 type crystal structure. For this reason, the material used for the soft magnetic underlayer of such a magnetic recording medium is selected so that the soft magnetic characteristics will not be deteriorated by the heating to the temperature of 500° C. or higher.
Recently, however, there are demands to further improve the medium SNR and the overwrite characteristic. Such demands are difficult to satisfy by the method of adding the grain boundary material to the magnetic layer, and the method of providing the soft magnetic underlayer.